The present invention relates to a lens system for a projection type television suitable for obtaining projection images of high quality.
Where the television screen is to be enlarged and the color cathode-ray tube of the type now available is made larger in size directly, there arise problems such as very high costs. Rather, a projection type television is more practical where an image on a cathode-ray tube is projected on the screen and enlarged through an optical system. However, in order to minimize the depth of a television set and effect a projection of a high definition image, it is necessary to use projection lenses of good characteristics.
For example, in U.S. Pat. Nos. 4,300,817 and 4,348,081 there are disclosed lens systems capable of realizing a brightness of 1.0 in terms of F-number using only three lenses. However, those lenses have a small field angle, which is about 50 degrees, so the projection distance becomes long and it is difficult to make the television set small in size.
On the other hand, in Japanese Patent Laid-Open No. 200216/1985 (U.S. Ser. No. 715,950 filed in March 1985) there is disclosed a lens system which can realize a wide field angle not smaller than 65 degrees though the number of lenses used is only three, thus greatly contributing to the reduction in size of the television set.
However, the above conventional lens systems are for the television screen according to the NTSC system which is now commonly adopted their performance, especially focusing performance, is unsatisfactory for projection reproduction of a television screen of high definition and high quality.
In the projection type television, reproduced images on three monochromatic cathode-ray tubes which function to reproduce only red, blue and green images respectively are projected in a superposed manner using an optical system, so it has generally been considered that chromatic aberration will not occur. Actually, however, chromatic aberration occurs because the phosphor of each monochromatic cathode-ray tube does not emit light at a specific wave length but rather emits light in a wavelength region of a certain range. In the lens systems disclosed in U.S. Pat. Nos. 4,300,817 and 4,348,081 and Japanese Patent Laid-Open No. 200216/85 aspherical lenses as used so spherical aberration is very small, resulting in chromatic aberration which is a great cause of deterioration in focusing performance. In the lens system disclosed in the Japanese patent laid-open 200216/85 it is intended to correct chromatic aberration by shortening the focal length, but the results are not satisfactory.
In connection with a high-quality image projection lens, a rear projection type display for Model 50 Hivision was made public in a 1986 TV Society National Meeting. This lens system comprises seven lenses using aspherical surfaces to reduce aberrations, including chromatic aberration, thereby realizing high focusing performance. However, due to a field angle of about 50 degrees, the projection distance is long and the depth of the set is large. Moreover, such a small field angle results in both the focal length and the lenses becoming larger, and this presents an obstacle to the reduction of cost.
According to the prior art, as noted above, it is impossible to simultaneously attain high focusing performance for high image quality, widening of field angle for reducing the depth of the set, and reduction in the size of lenses reduce cost at the same time.
List of prior applications:
U.S. Ser. No. 911,070 filed in September 1986 PA1 U.S. Ser. No. 948,251 filed in December 1986